CHAPTER XX. 

 STUDY OF THE PHYSIOLOGY OF BACTERIA. 



Of the environmental conditions determining the growth 

 of bacteria the following are the chief ones ordinarily deter- 

 mined : 



A. Temperature. — The optimmn temperature for growth 

 is usually about the temperature of the natural environment 

 and ordinarily one determines merely whether the organism 

 grows at body temperature (37°) and at room temperature 

 (20°) or not. For exact work the maximum, minimum and 

 optimum temperature must be ascertained by growing in 

 "incubators" with varying temperatures. 



A bacteriological incubator is an apparatus for growing 

 bacteria at a constant temperature. This may be any tem- 

 perature within the limits for bacterial growth. If tempera- 

 tures above that of an ordinary room are desired, some source 

 of artificial heat is needed. Electricity, gas or oil may be 

 used. A necessary adjunct is some device for maintaining 

 the temperature constant, a " thermoregulator" or "ther- 

 mostat." For lower temperatures a cooling arrangement 

 must be installed. For the great part of bacteriological work 

 only two temperatures are used, 20° so-called "room tem- 

 perature" (this applies to European "rooms" not to Amer- 

 ican) and 37° or body temperature. Incubators for 37° of 

 almost any size and style desired may be secured from sup- 

 ply houses and need not be further described. Figs. 139 

 and 140 illustrate some of the types. 



For use with large classes "incubator rooms" are to be 

 preferred. The author has one such room for 37° work with 

 200 compartments for student use which did not co§t over 

 $60 to install. 



The styles of incubators for lower temperatures, 20° and 

 below, are not so numerous nor so satisfactor\'. The author 



